Magnetic Resonance Imaging (MRI) is of great use for generating an image of the internal tissues of a human body. However, for persons who have an implantable medical lead implanted in their body, there are problems with induced currents in the implantable medical lead causing, in turn, heating of the lead, in particular at the distal tip of the lead. MRI is based on Nuclear Magnetic Resonance (NMR) for protons of hydrogen nuclei. It is well-known that all nuclei have spin which are randomly oriented. When a magnetic field is applied, the proton spins become either parallel or anti-parallel, and the energy levels are split into a higher level for the anti-parallel spin and a lower level for the parallel spin. The difference in energy between the two states is proportional to the magnetic field. Furthermore, the protons start precessing around the magnetic field direction with a precession frequency (Larmor frequency) which is proportional to the magnetic field, and with a precession angle, which is also called flip angle and is defined as the angle between the precession axis and the direction of the magnetic field. This precession angle in fact reflects the ratio between the energy levels. If an external pulsed radio frequency (RF) signal with Larmor frequency is applied, a few protons from the lower energy level with parallel spin will be excited to the higher energy level. This implies that the precession angle will change and the protons will precess in phase. The RF pulse width is typically of a few ms and is generally chosen for a time duration at which all protons are guaranteed to precess in phase. After some time (in the order of ms) these protons start to relax, that is the protons excited to the higher anti-parallel spin level will fall back to the lower parallel spin level, which implies that the precession angle falls back to the original value, and at the same time these protons will also de-phase. Both these processes will proceed with slightly different time constants, generally in the order of hundreds of ms.
A homogeneous medium can be easily identified by measuring the two time constants using this nuclear magnetic resonance (NMR) method. For an inhomogeneous medium, such as a human body, however, a gradient magnetic field is added on top of the static magnetic field to make every volume element unique. Along with accurate timing, the two time constants of the volume elements can be extracted and the individual volume element properties can be identified. This imaging method using the NMR principle is known as MRI.
In vitro MRI experiments have shown that an implantable medical lead acts like an antenna since the effective length of the lead is close to a multiple of the RF wavelength and thereby receives the pulsed RF signal of the MRI scanning equipment. The reception of the RF energy results in an RF wave propagating along the lead and heating the lead tip to an unacceptable level. Some other parts of the lead become heated as well, although not as much as the tip.
Referring to an in vitro set up, where a particular gel is used to simulate human tissue, the mechanisms for the RF energy transfer are identified as follows. As mentioned above, the precession frequency is proportional to the magnetic field, and more particularly 42.58 MHz/Tesla. Currently most MRI devices and systems operate at 1.5 Tesla (T), while 3 T MRI devices are expected to increase in the future. Thus, the frequency of the RF pulses, or RF wave, produced in a 1.5 T MRI device is about 64 MHz. The RF wave first passes through the boundary between the air and the gel. The RF wave undergoes a speed reduction from the speed in air v0 to a speed in the gel (human body) v1 due to the dielectric constant (∈) of the gel, where
      v    1    =                    v        0                    ɛ              .  The wavelength λ is also reduced by the same factor, i.e.
            λ      1        =                  λ        0                    ɛ              ,where λ0 is the wavelength in air and λ1 denotes the wavelength in the gel (human body). The dielectric constant of the human tissue on average is in such an order that the resulting wavelength in human tissue becomes close to the physical length of a typical implantable medical lead, e.g. in the order of half a meter. This transforms the lead to a good antenna. The RF energy is picked up by the outer conductor coil of the lead, and is then transferred to the inner conductor coil via the inter-coil capacitance. This coaxial structure of the lead in fact forms a transmission line, and the potential difference along the lead and between the outer and inner conductor coils cause the above-mentioned propagation. The RF energy is eventually transferred to the lead tip, causing heating of the tip.
This problem of lead tip heating has been addressed in the prior art, such as in US 2008/0033497 A1, where different solutions have been suggested. According to one solution, the inner and outer conductor coils are wound in opposite directions and they are interconnected at their ends. The purpose is to reduce the total current. According to another solution, RF blocking circuits are inserted. According to yet another solution the conductor coils are arranged so as to create resonance circuits.
WO 2007/047966 also aims at providing a solution to the problem of tip heating, however in a lead structure where the conductors are not provided in a coaxial structure with inner and outer conductor coils but arranged in parallel. Either the conductors are straight and parallel, individually and partially wound and parallel, or co-wound while still parallel. Capacitors are arranged to interconnect the conductors. The capacitors are arranged at regular or irregular distances from each other. By means of the capacitances a high impedance circuit is obtained, which, when appropriately tuned, reduces the coupling of the pulsed RF signal to the lead.
There is still a need for an implantable medical lead that is compatible with MRI systems, and in particular such an implantable medical lead that can be easily manufactured.